os: run more of TestExecutable on OpenBSD
os: run more of TestExecutable on OpenBSD

On OpenBSD, Executable relies on Args[0].  Removing the forgery on
that OS allows the rest of the test to run.

See #19453

Change-Id: Idf99f86894de5c702893791bc3684f8665f4019d
Reviewed-on: https://go-review.googlesource.com/46398
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

cloudflare/go,CAFxX/go,CAFxX/go,cloudflare/go,vsekhar/elastic-go,nawawi/go,codestation/go,nawawi/go,josharian/go,vsekhar/elastic-go,codestation/go,Cofyc/go,josharian/go,golang/go,codestation/go,vsekhar/elastic-go,akutz/go,polyverse-security/go,Cofyc/go,josharian/go,christopher-henderson/Go,Cofyc/go,codestation/go,Cofyc/go,nawawi/go,CAFxX/go,josharian/go,nawawi/go,polyverse-security/go,golang/go,vsekhar/elastic-go,cloudflare/go,akutz/go,polyverse-security/go,nawawi/go,polyverse-security/go,codestation/go,nawawi/go,nawawi/go,vsekhar/elastic-go,akutz/go,CAFxX/go,akutz/go,CAFxX/go,polyverse-security/go,golang/go,vsekhar/elastic-go,vsekhar/elastic-go,cloudflare/go,cloudflare/go,cloudflare/go,CAFxX/go,josharian/go,josharian/go,akutz/go,cloudflare/go,cloudflare/go,Cofyc/go,akutz/go,golang/go,golang/go,josharian/go,golang/go,polyverse-security/go,CAFxX/go,nawawi/go,akutz/go,josharian/go,golang/go,Cofyc/go,codestation/go,vsekhar/elastic-go,golang/go,codestation/go,codestation/go,polyverse-security/go,polyverse-security/go,Cofyc/go,Cofyc/go,CAFxX/go,akutz/go